Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the printing head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved over a transport distance, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the printing head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Generally, there is a mechanism for positioning a marker element in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots. Certain artefacts such as Moiré patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots. One source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as is the location of the marker elements in the printing head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the printing. A variation in drop velocity will also cause artefacts when the printing head is moving as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the way the printing medium is moved relative to the printing medium may result in defects which may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors. In fact, any geometrical limitation of the printing system can be a source of errors, e.g. the length of the printing head, the spacing between marking elements, the indexing distance of the printing medium relative to the printing head in the slow scan direction. Such errors may result in “banding” that is the distinct impression that the printing has been applied in a series of bands. The errors involved can be very small—the colour discrimination, resolution and pattern recognition of the human eye are so well developed that it takes remarkably little for errors to become visible.
To alleviate some of these errors it is known to alternate or vary the use of marker elements so as to spread errors throughout the printing so that at least some systematic errors will then be disguised. For example, one method often called “shingling” is known from U.S. Pat. No. 4,967,203 which describes an ink jet printer and method. Each printing location or “pixel” can be printed by four dots, one each for cyan, magenta, yellow and black. Adjacent pixels on a raster line are not printed by the same marking element in the printing head. Instead, every other pixel is printed using the same marking element. In the known system the pixels are printed in a checkerboard pattern, that is, as the printing head traverses in the fast scan direction a marking element is able to print at only every other pixel location. Thus, any marking element which prints consistently in error does not result in a line of pixels in the slow scan direction each of which has the same error. However the result is that only 50% of the marking elements in the printing head can print at any one time. In fact, in practice, each marking element prints at a location which deviates a certain amount from the correct position for this marking element. The use of shingling can distribute these errors through the printing. It is generally accepted that shingling is an inefficient method of printing as not all the marking elements are used continuously and several passes are necessary.
As said above, this kind of printing has been called “shingling”. However, printing dictionaries refer to “shingling” as a method to compensate for creep in book-making. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same marking element. Therefore, from here on and in what follows, the terms “mutually interstitial printing” or “interstitial mutually interspersed printing” are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
Another method of printing is known as “interlacing”, e.g. as described in U.S. Pat. No. 4,198,642. The purpose of this type of printing is to increase the resolution of the printing device. That is, although the spacing between marking elements on the printing head along the slow scan direction is a certain distance x, the distance between printed dots in the slow scan direction is less than this distance. The relative movement between the printing medium and the printing head is indexed by a distance given by the distance x divided by an integer.
For example, as illustrated in FIG. 1, a first part of a printing head 10 (e.g. the first quarter) prints first every so many columns, e.g. every fourth column. Then the printing head 10 is transported by one pixel pitch+(k1*marking element pitch), it is to be noted that k1 is an integer which may be zero. Then in the next pass the printing head 10 prints again every so many columns, e.g. every fourth column beginning with the second one, then the printing head 10 is transported one pixel pitch+(k2*marking element pitch), (k2 is an integer which may be zero). This procedure is repeated a number of times, e.g. a third time and a fourth time, after which the printing head 10 can be displaced by the remainder of the head length. The value of k (generally, ki) can be chosen freely.
If an image is printed using different sub-images with interlacing, often at least two different transport distances are necessary. In FIG. 1, 4 times interlacing is illustrated, i.e. the pixel pitch is ¼th of the marking element pitch. It can be seen from FIG. 1 that, in the example given, the transport distances are as follows:TD1=1 pixel pitch+2 marking element pitchTD2=1 pixel pitch+2 marking element pitchTD3=1 pixel pitch+2 marking element pitchTD4=1 pixel pitch+1 marking element pitchTD5=1 pixel pitch+2 marking element pitchTD6=1 pixel pitch+2 marking element pitch
TD4 is smaller than TD1 to TD3 in order to be able to write at every single pixel position.
For stability reasons, it can be advantageous to use only one physical transport distance.
In EP 01 014 299 a method for performing interlaced printing is described, in which method equal transport distances are used. In the method described, a number of requirements need to be met:    1. The number of sub-scan feeds (printing passes) in one feed cycle (i.e. before one head length is printed) needs to be equal to the marking element pitch k (expressed in pixels) multiplied by the number of mutually interstitial printing steps.    2. The marking element offsets F after the respective subs-scan feeds in one feed cycle assume different values in the range of 0 to (k−1), as many times as there are mutually interstitial printing steps in one feed cycle.    3. The average sub-scan feed amount (ΣL/(k*P)) is equal to the number of effectively used marking elements Neff (Neff=N/P), N being the number of used marking elements of the printing head.
The above conditions can be understood as follows. Since (k−1) raster lines are present between adjoining marking elements in the printing head, the number of sub-scan feeds required in one feed cycle is equal to k so that the (k−1) raster lines are serviced during one feed cycle and that the marking element position returns to the reference position (the position of the offset F equal to zero) after one feed cycle. If the number of sub-scan feeds in one feed cycle is less than k, some raster lines will be skipped. If the number of sub-scan feeds in one feed cycle is greater than k, on the other hand, some raster lines will be overwritten. Therefore, the first condition is required.
If the number of sub-scan feeds in one feed cycle is equal to k, there will be no skipping or overwriting of raster lines to be recorded only when the marking element offsets F after the respective sub-scan feeds in one feed cycle take different values in the range of 0 to (k−1). Therefore, the second condition is required.
When the first and the second conditions are satisfied, each of the N marking elements records k raster lines in one feed cycle. Namely N*k raster lines can be recorded in one feed cycle. When the third condition is satisfied, the marking element position after one feed cycle, that is, after the k sub-scan feeds, is away from the initial position by the N*k raster lines.
Satisfying the above first through third conditions thus prevents skipping or overwriting of raster lines to be recorded in the range of N*k raster lines.